Over the last two decades, staphylococcal infections have become important causes of human morbidity and mortality, particularly in hospitalized patients. Because of their prevalence on the skin and mucosal linings, staphylococci are ideally situated to produce infections, both localized and systemic. Debilitated or immunosuppressed patients are at extreme risk of systemic infection.
The staphylococcus species most frequently pathogenic in humans are Staphylococcus aureus and Staphylococcus epidermidis. Each species includes a number of serotypes. Both groups have developed resistance to antibiotics, the current treatment of choice.
In recent years, S. epidermidis has become a major cause of nosocomial infection in patients having treatments comprising placing implants into the body, such as cerebrospinal fluid shunts, cardiac valves, vascular catheters, and joint prostheses. S. epidermidis is also a common cause of postoperative wound infections and peritonitis in patients with continuous ambulatory peritoneal dialysis. One form of treatment for kidney failure entails the introduction of large volumes of peritoneal dialysis fluid into the peritoneal cavity, a treatment carrying a risk of frequent and recurrent infections.
Patients with impaired immunity and those receiving parenteral nutrition through central venous catheters are at high risk for developing S. epidermidis sepsis (C. C. Patrick, J. Pediatr., 116:497 (1990)). In particular, S. epidermidis has become a common cause of neonatal nosocomial sepsis, and is now the most common cause of bacteremia in the neonatal intensive care unit setting. Infections frequently occur in premature infants receiving parenteral nutrition, which can be a direct or indirect source of contamination. Such infections are difficult to treat for a variety of reasons. For example, resistance to antibiotics is common. In one study, the majority of staphylococci isolated from blood cultures of septic infants were multiply resistant to antibiotics (Fleer et al., Pediatr. Infect. Dis., 2:426 (1983)). Stimulation of the immune system provides little relief because such infants have impaired immunity resulting from deficiencies in antibodies, complement, and neutrophil function. Moreover, lipid infusion, which is now a standard ingredient of parenteral nutrition therapy, further impairs the already poor infant immune response to bacterial infection (Fischer et al., Lancet, 2:819 (1980)). Infection with S. epidermidis in these patients increases morbidity and mortality, and adds intensive care days that markedly increase medical costs.
Supplemental immunoglobulin therapy has been shown to provide some measure of protection against certain encapsulated bacteria, such as Hemophilus influenzae and Streptococcus Pneumoniae. Infants deficient in antibody are susceptible to infections from these bacteria, and thus, bacteremia and sepsis resulting from infection are common. When anti-Streptococcal and anti-Hemophilus antibodies are present, they provide protection by promoting clearance of the respective bacteria from the blood. In the case of antibody specific for staphylococcus, the potential use of supplemental immunoglobulin to prevent or treat infection has been much less clear.
Early studies of staphylococcal infections focused on the potential use of supplemental immunoglobulin to boost peritoneal defenses, such as opsonic activity, in patients receiving continuous ambulatory peritoneal dialysis. Standard intravenous immunoglobulin (IVIG) was shown to have lot to lot variability for opsonic activity to S. epidermidis (L. A. Clark and C. S. F. Easmon, J. Clin. Pathol., 39:856 (1986)). In this study, one third of the tested IVIG lots had poor opsonization with complement, and only two out of fourteen were opsonic without complement. Thus, despite the fact that the IVIG lots were made from large plasma donor pools, good opsonic antibody specific for S. epidermidis was not uniformly present. Treatment with such immunoglobulin would therefore not provide protection against Staphylococcal infection. This study did not examine whether IVIG could be used to prevent or treat S. epidermidis infections or bacterial sepsis.
Recent studies have associated coagulase-negative staphylococci, such as S. epidermidis, as the most common species causing bacteremia in neonates receiving lipid emulsion infusion (Freeman et al., N. Engl. J. Med., 323:301 (1990)). The neonates had low levels of opsonic antibody to S. epidermidis despite the fact that sera had clearly detectable levels of IgG antibodies to S. epidermidis peptidoglycan (Fleer et al., J. Infect. Dis., 2:426 (1985)). This was surprising because anti-peptidoglycan antibodies were presumed to be the principal opsonic antibodies. Thus, while suggesting that neonatal susceptibility to S. epidermidis might be related to impaired opsonic activity, these studies also suggested that many antibodies directed against S. epidermidis are not opsonic and would not be capable of providing protection when given passively to neonates. Moreover, the antigens responsible for inducing opsonic antibodies were not identified.
Recently, an antigen binding assay was used to analyze IgG antibody to S. epidermidis in patients with uncomplicated bacteremia and in patients with bacteremia and endocarditis (Espersen et al., Arch. Intern. Med., 147:689 (1987)). This assay used an ultrasonic extract of S. epidermidis to identify S. epidermidis specific IgG. None of the patients with uncomplicated bacteremia had IgG antibodies specific for S. epidermidis. These data suggest that IgG does not provide effective eradication of S. epidermidis from the blood. In addition, 89% of bacteremic patients with endocarditis developed high levels of IgG to S. epidermidis. In these patients, IgG was not protective since high levels of IgG antibody were associated with serious bacteremia and endocarditis. Based on these studies, the protective role of IgG in S. epidermidis sepsis and endocarditis was questionable, especially in the presence of immaturity, debilitation, intralipid infusion, or immunosuppression.
The role of antibody in immunity to S. epidermidis has also been studied in animal models (Kojima et al., J. Infect. Dis., 162:435–441 (1990); and Yoshida et al., J. Appl. Bacteriol., 47:299–301 (1979)). Animal studies that demonstrated immunoglobulin protection against staphylococcal infections have shown strain specificity by enzyme-linked immunosorbent assays (ELISA). These studies utilized normal adult mice having a mature immune system in protection studies, and therefore do not mimic the disease observed in humans. Studies using mature animals with normal immunity typically comprise administering to the animals unusually virulent strains or overwhelming-challenge doses of bacteria. This does not mimic infection in humans because human patients are generally immunologically immature or debilitated. Human patients can also have somewhat indolent infections with low virulence pathogens, such as S. epidermidis, with death usually attributable to secondary complications rather than the bacterial infection. Models using unusual strains or overwhelming bacterial doses generally induce rapid fulminant death.
These factors are important since antibodies generally work in concert with the host cellular immune system (neutrophils, monocytes, macrophages, and fixed reticuloendothelial system). The effectiveness of antibody therapy may therefore be dependent on the functional immunologic capabilities of the host. To be predictive, animal models must closely mimic the clinical condition in which the infection occurs and capture the setting for therapy.
Prior animal studies have yielded inconsistent results. One animal model used an unusually virulent strain of S. epidermidis. Infected mature mice developed 90 to 100% mortality within 24 to 48 hours (Yoshida et al., Japan. J. Microbiol., 20:209 (1976)). Antibody to S. epidermidis surface polysaccharide was protective in these mice, with protection occurring for an IgM fraction but not an IgG fraction (K. Yoshida and Y. Ichiman, J. Med. Microbiol., 11:371 (1977)).
This model presents a pathology very different from that typically seen in infected patients. Intraperitoneally-challenged mice developed symptoms of sepsis within minutes of receiving the injection and died in 24 to 48 hours. This pathology is not observed in staphylococcus-infected humans. The highly virulent strain of S. epidermidis may represent an atypical type of infection. Moreover, isolates of S. epidermidis from infected humans did not kill mice in this model.
In 1987, animal studies were extended to include the evaluation of antibodies in human serum against selected virulent strains of S. epidermidis (Ichiman et al., J. Appl. Bacteriol., 63:165 (1987)). In contrast to previous data, protective antibody was found in the IgA, IgM, and IgG immunoglobulin fractions. A definitive role for any single class of immunoglobulin (IgG, IgM, IgA) could not be established.
In this animal model, mortality was determined for normal adult mice. Death was considered to be related to the effect of specific bacterial toxins, not bacteremia sepsis (Yoshida et al., Japan J. Microbiol., 20:209 (1976)). Most clinical isolates did not cause lethal infections, and quantitative blood cultures were not done. This study provided little insight as to whether antibody could successfully prevent or treat S. epidermidis sepsis in immature or immunosuppressed patients.
In a later animal study, serotype specific antibodies directed against S. epidermidis capsular polysaccharides were tested. Results showed that serotype-specific antibodies were protective, but that each antibody was directed against one particular serotype as measured by ELISA (Ichiman et al., J. Appl. Bacteriol., 63:165 (1987)). Protection was equally serotype specific. Protection against heterologous strains did not occur. In addition, it was concluded that protection was afforded by the IgM antibody.
In short, there has been no compelling evidence that IVIG, which contains only IgG, could be effective to treat and prevent S. epidermidis infections or sepsis, particularly where patients are immature or immune suppressed, or where multiple S. epidermidis serotypes are involved. Thus, for example, a recent and extensive review of the pathogenesis, diagnosis, and treatment of S. epidermidis infections does not include immunoglobulin as a potential prophylactic or therapeutic agent (C. C. Patrick, J. Pediatr., 116:497 (1990)).
An animal model that mimics human S. epidermidis infections has not been developed, particularly for humans that are immature or immune suppressed. This is critical because these patients have low levels of complement as well as impaired neutrophil and macrophage function. Thus, even if opsonic activity of immunoglobulin may appear adequate under optimal conditions in vitro, protection may not occur in patients such as newborn babies or cancer patients. Moreover, previous models are unsatisfactory in that they used animals which did not possess similar risk factors as the typical high-risk human patient.
Although coagulase negative staphylococci (CNS) are significant as nosocomial pathogens, no effective method to prevent CNS infections has been developed. The current preferred treatment of choice for the prevention and cure of staphylococcal infections in humans is antibiotic therapy. Although new antibiotics are constantly being developed, it has become increasing clear that antibiotic therapy alone is insufficient. Data regarding passive vaccinations with immunoglobulin is at best unclear. The animal models on which this therapy has been attempted bear little relationship to human infections and as yet, have produced no definitive solutions. In summary, there is a need in the art for an effective treatment for staphylococci infections.